I just upgraded my large polk monitor bookshelves to Tsi400 floorstanders for a steal. As for my exisiting Center: Polk Monitor 15C vs (TSi series) CS10 are nearly identical on paper. Would I notice any difference with a CS10? Thanks

From everything I have seen the difference would be marginal. I have had the CS10 and TSI400's since they were the new model line and really enjoy them, however the largest reason to switch would likely be for aesthetic reasons and to ensure good timbre matching as the overall sound differences aren't likely to be too significant.